Clarifiers are tanks where solids-liquid separation occurs due to gravity. They are used in chemical processing, as well as in nearly every wastewater treatment plant. This invention presents a facility and method for solids (or sludge) collection and withdrawal in clarifiers.
The capacity of a clarifier is highly dependent on the efficiency of sludge draw-off facility. A good design and application of a sludge withdrawal facility should provide fast sludge removal and less sludge dilution to avoid a high sludge blanket, which would harm the effluent quality in clarifiers under high solids loading conditions.
As shown in FIGS. 1 and 2 there are two types of sludge collection and withdrawal facilities, Rotational Sludge Scrapers and Rotational Sludge Suction Tubes.
The conventional sludge scraper system consists of rake arms attached with several sludge collection blades with an attack angle between blade axis and rake arm, rake arm driver and a sludge withdrawal hopper around the center column. In recent years, the sludge withdrawal hopper occasionally replaced by Sludge Collection Box (U.S. Pat. No. 5,219,470). The rake arm extends from center column to clarifier side-wall and is able to rotate. The combination of the force due to the rotation of rake arm (along the tangential direction) with the friction between sludge and blade results in the momentum driving the sludge toward the center hopper, where the sludge is removed out of the tank. The shape of the blade used for picking up the sludge is commonly flat or spiral.
In the system of sludge suction tubes, the rotational rake arms are extended from center column to clarifier side-wall and attached with several sludge suction tubes. The suction heads (upstream ends of suction tube) are evenly distributed along the rake arms (in the radial direction). The rake arms together with suction heads rotate along the tank bottom. The downstream ends of suction tube are connected into two separated return sludge boxes normally rectangular in shape, which are typically hung on both sides of a rotatable cage surrounding the influent column. There are many alternatives and applications for the connection between the sludge suction tubes and the suction driving force (usually a pump), such as Rotational Center Well (U.S. Pat. No. 4,193,877) and Sludge Collection Box (U.S. Pat. No. 5,219,470).
Another similar system to the sludge suction tubes described above is called xe2x80x9cUnitubexe2x80x9d or xe2x80x9cTow-Broxe2x80x9d, which consists of a rotatable rectangular (or circular) shaped pipe and a manifold around the center column. Many small suction orifices are distributed along the pipe, which is normally constructed in plate steel for structure stability and extended from clarifier center to side-wall. The rotatable pipe is of tapered design with the cross-section decreasing from the center of the tank to the outer tip for a uniform sludge withdrawal. The xe2x80x9cUnitubexe2x80x9d rotates to cover entire tank bottom in a certain period.
Using currently available sludge draw-off facilities, either sludge has to be transferred to the withdrawal point (center sludge hopper or sludge collection box) or withdrawal points (suction heads attached on rake arms or suction tubes) have to be rotated to approach the sludge. In both cases, there is significant distance between sludge and withdrawal points.
The Multiple Stationary Sludge Draw-Off Tubes (MSSDOTs) consists of the following 4 major components:
1. Multiple Sludge Collection Tubesxe2x80x94As shown in FIG. 3, several sludge collection tubes are located in the tangential direction at different radii in a circular clarifier. The cross section of each sludge collection tube could be any shape (most likely circular, square or rectangular, etc.). In square clarifiers, the inner sludge collection tubes still can form circular rings but the outer sludge collection tubes should go along the side-wall direction to cover the four corner areas. In principle, as the radius of a clarifier increases, more sludge collection tubes are needed. The sludge collection tubes could be directly laid on the clarifier bottom or also could be laid in gutters on the bottom. Small orifices are distributed along the tangential direction on each sludge collection tube (see FIGS. 4 and 5). The size of small orifices and the distance between two orifices along the tangential direction could be varied to allow flows through the orifices into the collection tube to be more evenly distributed. The orientation of these orifices should always be toward the majority of sludge inventory near the tank bottom.
2. Multiple Sludge Transfer Tubesxe2x80x94As shown in FIGS. 4 and 5, on each sludge collection tube, several sludge transfer tubes (most likely 4) located along the tank""s radial direction, are used to connect one sludge collection tube with the center connection ring. In a case with 4 sludge transfer tubes, the angle between two adjacent sludge transfer tubes is 90 degrees (see FIG. 4). In normal operation, the return sludge flow from each sludge collection tube should be evenly distributed among all of sludge transfer tubes. At the downstream end of each sludge transport tube, a valve should be installed to provide the system with the ability to control flows from different sludge transfer tubes. By utilizing the valves and pipe lines mentioned above, the entire sludge return flow of the clarifier could be withdrawn through a single sludge transfer tube to flush out any possible blockages in any part of the piping system.
3. Center Connection Ringxe2x80x94a central connection ring located on clarifier center (at any level in clarifier or underneath clarifier depending on design of return activated sludge flow pumping system) is used to assemble all of sludge transfer tubes together at the upstream end of the ring. The downstream end of central connection ring is connected with the sludge withdrawal pump as shown in FIGS. 4 and 5.
4. Sludge Withdrawal Driving Forcexe2x80x94A sludge withdrawal pump connected with the downstream end of central collection ring can be used to provide the driving force of sludge withdrawal. The driven force of sludge removal flow can also be provided by the hydraulic head differences between the water surface in clarifier and the sludge transport system that is used to transfer sludge from the secondary clarifier back to the aeration tanks.
This invention also includes a simplified MSSDOTs, which only uses multiple stationary sludge transfer tubes as shown in FIG. 8. In this case the small orifices for sludge suction are distributed on the wall of sludge transfer tubes along the tank""s radial direction.
In circular clarifiers (or square) using sludge collection scrapers together with conventional center sludge withdrawal hopper or sludge removing box [Bradley, et al (1993), U.S. Pat. No. 5,219,470], the length of each rake arm attached with several scraper blades extends from the clarifier center to the side-wall in order to collect sludge along the entire clarifier radius. The average sludge transfer distance picking up and pushing toward the tank center by scraper blades is half of the clarifier radius.
Using Multiple Stationary Sludge Draw-Off Tubes to replace a conventional center sludge withdrawal hopper or sludge removing box in circular (or square) clarifiers with sludge collection scrapers, the average sludge transfer distance can be reduced from half the clarifier radius to the distance between two collection tubes. The sludge can be directly drawn-off from where solids are settled rather than be transferred to the center hopper and then moved out. Therefore, the sludge transport speed in clarifiers with MSSDOTs is much faster than that in clarifiers with a center sludge withdrawal hopper or a sludge removing box. Thus, clarifier capacity could be significantly increased.
In many existing secondary clarifiers the rotation sludge suction tubes or Unitube sludge removers (Tow-Bro) are applied. In those clarifiers with sludge suction tubes, the rotation rake arms with several sludge tubes attached on each of the arms are rotated along the tangential direction. In the design of Tow-Bro, one or two rotation sludge withdrawal pipes extend from the center of the clarifier to the side-wall. On the rotational sludge withdrawal pipes many small sludge suction orifices are distributed along the pipe axis. Both the rotational sludge suction tubes and Tow-Bro are able to evenly distribute the sludge draw-off flows thus avoid significantly sludge dilution along the clarifier""s radial direction. However, in the square (or circular) clarifiers with both existing rotational sludge collection and removing facilities, sludge draw-off flows along the tangential direction are unevenly distributed. All of sludge flow is withdrawn within a very narrow strip where the suction arms are fixed. Using this type of sludge withdrawal equipment, significant flow convection along the tangential direction is generated due to the flow compensation from the sections with no sludge withdrawal into the sludge withdrawal section.
In addition to the problems of a rotational sludge collection or suction system mentioned above, the high tip speed due to the rake arm (or Tow-Bro) rotation could often disturb and re-suspend the settled sludge near clarifier side-wall.
Using MSSDOTs to replace rotation rake arms with suction tubes (or Tow-Bro), the sludge withdrawal occurs in the tangential direction at all times, not just when the rake arm passes by. Therefore, uniformly distributed sludge removal can be achieved on both clarifier radial and tangential directions, which provides the higher sludge draw-off efficiency (thus lower sludge inventory in the tank) and lower disturbance on the flow regime in the clarifiers.